The use of drugs, such as bromouracil derivatives for enhancing the radiation sensitivity of malignant cells is a potentially useful technique in the treatment of cancer. The aim of this study is to determine the mechanism of the sensitizing effect. The key to this in- vitro study of radiation sensitization is the use of single crystal solid solutions (mixed crystals) which contain a small percentage of sensitizer molecules. Cryogenic techniques will be used to maintain the crystals at low temperature (4.2 degrees K or lower) during x- irradiation thereby stabilizing the primary stage of the radiation damage process. Electron-nuclear double resonance (ENDOR) spectroscopy will be used to identify the molecular changes (e.g., oxidation or reduction) induced by ionizing radiation. Subsequent steps in the radiation damage process can be observed on warming the crystal. The search for potentially more effective drugs to be used in conjunction with radiation therapy could be expedited by this approach.